1. Field of the Invention
The present invention relates to an integrated circuit package.
2. Description of Related Art
Integrated circuits are sometimes housed within a ceramic substrate that is sealed with a lid. The ceramic substrate typically has a plurality of outer surface leads/pins located on the bottom of the package. The outer surface leads are soldered/inserted to corresponding conductive features of a printed circuit board.
The outer leads/pin are coupled to inner surface pads located within an inner cavity of the ceramic substrate. The inner cavity also contains the integrated circuit which has a plurality of corresponding outer surface pads located on the top surface of the die. The outer pads of the die are coupled to the inner substrate surface pads by either TAB tape or conventional wire bonding techniques.
As shown in FIG. 1, integrated circuit dies 2 are typically rectangular in shape and have the outer surface pads 4 located along each edge of the die. The inner surface pads 6 of the substrate 8 are also located in a corresponding rectangular pattern about the integrated circuit.
Ceramic packages are typically constructed by attaching the surface pads and accompanying interconnective circuitry to an uncured ceramic material commonly referred to as"green tape". The tape and conductive material are then fired and cooled to create a hard ceramic substrate. Heating and cooling the ceramic and conductors induces an expansion and contraction of the two materials. The ceramic has a different coefficient of thermal expansion than the conductors so that each material grows and contracts at a different rate. Growth of the ceramic material originates at the center of the package and extends outward in a radial direction.
As shown in FIG. 2, the inner surface pads 6 may move to a different position during the firing process of the ceramic. The pad movement is somewhat unpredictable. Additional spacing between the pads must therefore be provided to compensate for the variations in pad position. This additional spacing limits the pitch of the pads. Additionally, any subsequent thermal expansion of the final assembled package will create a corresponding radial growth of the ceramic substrate and may induce shear stresses in the bond joint of the pads and bending moments of the bonding wires or TAB leads. It would be desirable to provide a package that provided a more accurate inner pad location and which reduced the stresses on the pad joints.